All around us we hear and read articles which suggest that soft drinks or carbonated beverages are acidic and damage our body but we still consume them (or atleast a significant proportion of us do). Why doesn't consuming such drinks during a bacterial infection in stomach help to kill all the bacteria?
There is insufficient evidence to claim that soft drinks can kill microbes in the human gut; sugar in them could actually stimulate their growth.
Natural acidity of the gastric juice with pH 1.5-3.5 kills a lot of microbes (Medline Plus, Journal of Medical Microbiology). The pH of common soft drinks is 2.5-3.4 (PubMed, Fig.1). So, drinking soft drinks does not likely make the gastric juice more acidic.
According to one in vitro study, soft drinks can help to kill some bacteria (ResearchGate, 2015). In another in vitro study, they did not observe any antimicrobial activity of Coca Cola (Journal of Food Protection, 2007).
I don’t know if soft drinks can help ward off bacterial infections in the stomach, however if you’re not taking antibiotics, other acidic compounds can be helpful for that. One of these is apple cider vinegar, you will find many videos on youtube about it. Another is betain-HCL, which is actually hydrochloric acid, the same acid we have in our stomach but it’s more expensive than ACV.
There is a big misconception regarding stomach acid problems and their treatment, and people should first check whether they have low or high acid level in their stomach, as both have almost the same symptoms. You can do that with a simple bicarbonate test.
I was diagnosed with mild GERD (gastroesophageal reflux disease), and I was surprised to learn that, contrary to popular belief, this common digestive problem does not arise from excess acid in the stomach but on the contrary from having too low acid, which causes the valve that we have at the bottom of our esophagus, called LES, to relax, so that the acidic content of the stomach goes up and reaches the delicate mucosa of the esophagus triggering the burning sensation, especially after a heavy meal.
Having low acid is something you want to avoid not only because you are more prone to digestive problems due to improperly broken down proteins, but also to infections - our food is not 100% sterile, bacteria are everywhere and even after washing or cooking there will always be some in our food, they don’t create problems to us simply because they get killed in the stomach.
The worst part of the GERD epidemics in the Western world due to excess and bad eating, is that doctors prescribe you acid reducers (so called proton pump inhibitors, they go with various brand names that contain omeprazole, lansoprazole, etc.) which further lower the acid content of the stomach. Doctors should know that, right? Well this drugs put you at higher risk of infections, given that in the long term they can lower the acid content in the stomach down to 5% of the normal level! Which is like inviting bacteria to a party. As if this were not enough, once you take them is very difficult to stop and get rid of their side effects, as there will be a rebound effect after a few days, with symptoms even worse than before you started the treatment, so you are literally bound to them. The only way to get rid of them is decreasing the dosage very slowly over a long period of time, usually months, while taking substitutes.
My doctor didn’t give me acid reducers, instead he told me that if I like soft drinks I can try coke zero, which has zero calories but still retains the acidity necessary to raise the stomach acid and help with digestion, and it works! Later I found that the same effect is given by apple cider vinegar and also vitamin C, aka ascorbic acid, and now I tend to prefer the vitamin over the rest.
Carbonic acid (H2CO3) itself in the human body is not stable and falls apart to water and carbon dioxide. When you consume anything by eating/drinking it and swallowing it via the gastro-instestinal tract, it does not enter your bloodstream directly but instead is filtered by various organs first and in many cases undergoes a transformation (First pass effect). In this case the main organ dealing with the drink would be the kidney.
So to answer your question more concisely: it is not stable, and it can not pass the apical cell membranes freely (especially in the proximal kidney tubuli). This is done by the body on purpose because if it could pass freely it would disrupt the pH balance (more specifically the bicarbonate buffer system) of the body. There are different mechanisms in the body that regulate this (look up any physiology textbook).